1. Field of the Invention
This invention relates generally to subscriber line interface circuits, and more particularly, to a generic line impedance matching circuit using decomposed configurable transfer functions.
2. Description of the Prior Art
Impedance matching is a very important function of any equipment that is attached to copper twisted pairs. A line-matched impedance helps control reflections and echoes in the line which then helps maintain good quality of service.
Previous impedance matching functions have been performed using hybrid transformers and a discrete matching impedance, and more recently, using Subscriber Line Interface Circuits (SLICs) with op-amps and a discrete matching feedback impedance. These methods have required large discrete components and large amounts of board space. The large number of different standardized line impedances (more than 24), and the need for custom line impedance matching make the design of a generic integrated impedance matching circuit very difficult and costly. A traditional SLIC implementation is illustrated in FIG. 1(a). The SLIC 10 provides the Tip and Ring function as well as the sense function required to perform an impedance match at the Tip Ring via the impedance matching op-amp network 20. This traditional implementation necessitates the requirement for a new op-amp network 20 for each different line impedance. The traditional op-amp impedance matching circuit (network) 20 can be represented as a transfer function given by H(S) as depicted in FIG. 1(b).
In view of the foregoing, it would be desirable and advantageous to provide a simple and inexpensive scheme to reuse the same circuit topology for implementing an impedance match for a diverse number of line impedances.